Digital subcarrier cross-connect switching

ABSTRACT

The present invention uses digital subcarrier cross-connect switching to accomplish various network processes more efficiently. These processes include interconnecting network components, and performing optical and optoelectronic add/drop operations.

RELATED APPLICATIONS

The present U.S. non-provisional patent application is acontinuation-in-part and claims priority benefit of an earlier-filednon-provisional patent application titled DIGITAL SUBCARRIER OPTICALNETWORK UTILIZING DIGITAL SUBCARRIER CROSS-CONNECTS WITH INCREASEDENERGY EFFICIENCY, Ser. No. 13/330,647, filed Dec. 19, 2011, which, inturn, is related to and claims priority of an even earlier-filedprovisional patent application titled POWER EFFICIENT OPTICAL NETWORKCROSS-CONNECT BASED ON FREQUENCY-DIVISION MULTIPLEXING AND RF SWITCHING,Ser. No. 61/424,581, filed Dec. 17, 2010. The identified earlier-filedapplications are hereby incorporated by reference into the presentapplication as though fully set forth herein.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates generally to cross-connect switches asused, for example, in interconnecting network components and inperforming optical and optoelectronic add/drop operations.

2. Background

Optical networks are connected through optical fibers with elementscapable of providing optical channel transport, multiplexing, routing,management of the network, supervision, and redundancy forsurvivability. Many telecommunications and data carriers around theworld are increasingly using Optical Transport Networks (OTN) for theirlong-haul and metro-area networks. OTN is growing faster thanSynchronous Optical Networking and Synchronous Digital Hierarchy(SONET/SDH) and has the potential of boosting bandwidth and increasingnetworking functionality.

Optical networks utilize optical fibers and lasers or highly coherentlight from light-emitting diodes to transfer multiple digital bitstreams of data over the network. SONET/SDH was originally designed toreplace Plesiochronous Digital Hierarchy (PDH) which was used totransport large amounts of telephone calls and data traffic over thesame fiber without requiring synchronization across the multiplexedtributary channels. PDH used circuit-switching and was spectrallyefficient only if the sources of the tributary channels weresynchronized. However, as these optical networks continued to grow, sodid the traffic on them. SONET and SDH, a superset of SONET, weredeveloped to support real-time, uncompressed, circuit-switched,digitally encoded voice and data. SONET/SDH allows for simultaneouslytransporting many different circuits (tributaries) of differing originusing a single framing protocol, and is ideal for transportingAsynchronous Transfer Mode (ATM) frames, Internet protocol (IP) packets,or Ethernet frames. Generally, a frame is a group of data bits in aspecific format (ATM, Ethernet, IP and others) with a flag at thebeginning and the end of the data bits to define the individual frame.

The message protocols transported by SONET and SDH are similar with afew exceptions. SONET is typically used in North America whereas SDH iswidely used throughout the world. The protocol of SONET/SDH is a time(byte) division multiplexed structure wherein a header is interleavedbetween the data to permit the encapsulated data to have its own uniqueframe rate and be present within the SONET/SDH frame structure and rate.The protocols buffer data during transit for at least one frame beforesending. This buffering allows for multiplexed data to move within theoverall framing (transmission) to compensate for different frame rates.The protocol becomes more complex based on when and where in the datastream padding is needed and at what level of the multiplexingstructure.

In optical networks, SONET/SDH routers and add/drop multiplexers andcross-connectors have relatively high power consumption, and withincreased demand for these networks for industry, public works, school,and residential use, energy usage increases. The networking community'senergy saving object is becoming more important now that Internettraffic is expected to continue steep growth driven by videoapplications and cloud computing advances.

Energy consumption is a consideration in designing communicationnetworks and subsystems including hardware, routers and add/dropmultiplexers, and cross-connectors. For example, Internet Protocol (IP)routers can lower their packet processing rate when traffic volume islow to reduce energy consumption in both optical and electricalnetworks. All-optical Wavelength Division Multiplexing (WDM) networkscan be made more energy efficient by bypassing theoptical-electrical-optical conversion at the intermediate opticalcross-connection nodes. One layer of the communication networks whereincreased energy efficiency is desirable in current and future networksis in the third network layer, the OTN. The OTN layer is often usedbetween the IP and the WDM layer to provide sub-wavelength capacity tothe links of routers. Present day OTN solutions perform similar toSONET/SDH and perform digital time division multiplexing of multiplesub-wavelength channels to fill out the entire wavelength of a channel.Each sub-wavelength channel is individually routed using DigitalCross-Connects (DXC), and each DXC requires approximately 10 Watts per10 Gigabits of carried data to perform transport functionalities usingcurrent technology. Similar levels of energy consumption take place inthe OTN add/drop multiplexers, where tributary signals from the end-userare injected into or extracted from the OTN network. As the energyconsumption of telecommunications networks is forecast to grow manifolddue to the rapid increase of traffic volume in broadband networks,combined with the expectation of higher energy prices and increasingconcerns about global warming, finding energy-efficient solutionsbecomes an important issue for telecommunications networks.

At the IP layer, energy-aware packet forwarding techniques suggest thatsmaller IP packets increase the energy consumption of routers, sooptimizing the size of IP packets can make routers more energyefficient. However, reducing switching delay and lowering energyconsumption need to be carefully balanced. New network architecturecomprising two parallel networks have been proposed. A “super-highway”network using pipeline forwarding for IP packets would be used inconjunction with the current Internet which carries traditional trafficand signaling between routers that set up synchronous pipes insuper-highway networks. The super-highway would carry traffic that haspredictable patterns and require high bandwidth.

In WDM networks, high energy consumption originates from the opticalnetwork equipment which is used for traffic grooming. Hence,energy-efficient traffic grooming, which reduces the number of requiredlightpaths, considerably increases energy savings. Other approaches toreduce energy consumption include using Routing and WavelengthAssignment (RWA) heuristics that minimize the number of lightpathinterfaces, and using digital signal processing for wavelengthtranslation of the optical frequencies of each specific wavelength onthe optical fiber when required. However, this process may be costprohibitive due to the expense of the optical equipment based onexisting technology needed to create the wavelength translation. Otherpossible solutions include reducing energy consumption of each networkoperation by performing dynamic traffic grooming over time.

Current telecommunications networks are based on an architectural modelinvolving three classes of network domains: core, metro, and access. Incore networks, efforts to reduce energy consumption can be divided intotwo categories: energy-efficient network design and energy-efficientnetwork operations. The energy consumption of IP routers, EDFAs, andtransponders is jointly minimized for an IP-over-WDM network byutilizing Mixed Line Rates (MLR). Likewise, shutting down idle networkelements saves energy. To identify the maximum number of idle nodes andlinks while still supporting the ongoing traffic, heuristics and MixedInteger Linear Program (MILP) models can be used to reduce the powerednodes (or equipment) during off-peak hours and during trafficfluctuations throughout the day. Similarly, idle line cards can be shutdown when traffic load is low, while keeping the physical topologyinvariant or with the minimum required level of connectivity, to reducepower needs. “Green Routing” has been proposed which uses energyconsumption of network equipment as the optimization objective. Also,greater attention is being paid to renewable energy. One idea to reducecarbon footprint is to establish core servers, switches, and datacenters at locations where renewable energy can be found, and then toroute traffic to the “green areas”.

Wireless-Optical Broadband Access Network (WOBAN) is a novel accessarchitecture, and can provide high-bandwidth services. Energy savings inthe optical part of WOBAN by sleeping mechanism has been studied, andenergy-efficient design of a unidirectional WDM ring network has beeninvestigated.

Energy-efficiency is a major problem for data centers, which are vitalto support current data applications. Optical networks play an importantrole in both data center inter- and intra- connections. An approach toreducing the energy consumption of high-speed intra-connection (insidedata centers) links has been. Load distribution across data centers indifferent locations is also related with power-conservation. How tooptimally distribute requests has also been studied.

Solutions based on analog Frequency Division Multiplexing (FDM) werewidely used in the pre-SONET/SDH era, to multiplex transport channelstogether using spectral diversity in coaxial and fiber cables. Althoughanalog RF/microwave Subcarrier Multiplexing (SCM) in fiber optics isstill in use to carry radio signals between antennas and base station,FDM technologies in digital transport networks were abandoned due inpart to their relatively low spectral efficiency when compared to TimeDivision Multiplexing (TDM) and synchronous transmission techniques,such as SONET and SDH. Another problem of traditional FDM (or SCM),being analog systems, is their susceptibility to accumulated waveformdistortion and crosstalk. For these reasons analog FDM is not acompetitive solution for large-scale optical networks.

SUMMARY OF THE INVENTION

The present invention provides reduced power dissipation at the OTNlayer by utilizing Digital Subcarrier Multiplexing (DSCM) technologyexecuted on a DSCM add/drop multiplexer and cross-connect architectures.DSCM has advantages for wireless communications due to its high spectralefficiency and robustness against signal corruption. With advances indigital CMOS electronics and the increased use of optical networks, thepresent invention can be used to implement Digital SubcarrierCross-Connects (DSXCs) which perform the same transport functions of theOTN layer while significantly reducing energy consumption in thenetwork. Energy consumption is also reduced by efficiently designing thenetwork routing and resource (the subcarriers) assignment algorithmscross-connect functionalities, including routing frequency assignment(RFA) algorithms.

The DSCM cross-connect architecture of the present invention may also beused to lower the power consumption of existing communication networksby introducing it in various network segments. For example, packettransport networks make use of Multiprotocol Label Switching-TransportProtocol (MPLS-TP) routers to create virtual circuits and these networksconsume power at the approximate level of OTN. The reduction in powerconsumption offered by the DSXC layer may assist in the expansion ofcommunication, Internet, and education services to economicallydepressed area. Likewise, these low power-consuming DSXC nodes can bepowered by renewable energy such as solar cells or wind generators.

OFDM is an example of DSCM which utilizes orthogonality between adjacentchannels so that no guard band is required between adjacent channels,which maximizes optical bandwidth efficiency. Nyquist WDM (N-WDM) isanother example of DSCM which uses sharp-edged Nyquist filters tominimize the spectral overlap between adjacent subcarrier channels so asto minimize the crosstalk between them. The DSXCs of the presentinvention have numerous applications, including, for example, ininterconnecting network components and in performing optical andoptoelectronic add/drop operations.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings constitute a part of this specification and includeexemplary embodiments of the disclosed subject matter illustratingvarious objects and features thereof, wherein like references aregenerally numbered alike in the several views.

FIG. 1 (PRIOR ART) is a diagram of three layers of a prior art networkusing an IP layer, an OTN Layer, and a WDM layer;

FIG. 2 (PRIOR ART) is a block diagram representing a prior art digitalcross-connect switch using TDM;

FIG. 3 (PRIOR ART) is a block diagram illustrating the typical powerusage of a prior art electronic digital cross-connect;

FIG. 4 is a block diagram illustrating the typical power usage of anoptical circuit switch of the present invention;

FIG. 5 is an illustration of a three layer architecture the presentinvention comprising the IP layer, a DSXC layer, and the WDM layer;

FIG. 6 is an illustration of a digital subcarrier cross-connectarchitecture and coherent transceivers of the present invention;

FIG. 7 is a graphical representation of the optical spectra of OFDMsignals with C1, C5, and C10 subcarrier channels each carrying a 2 GbpsQPSK signal;

FIG. 8 is a graphical representation of a system Bit Error Rate (BER)versus optical-to-noise-ratio for the signals carried in subcarrierchannels C1, C5, and C10;

FIG. 9 is a graph of BER values for simultaneous detection of channelsC6 through C10 when a local oscillator (LO) is set at the centralfrequency of channel C9;

FIG. 10 is a graph of results when the local oscillator wavelength isset to the center of the signal optical carrier in an attempt to detectall ten subcarrier channels;

FIG. 11 is a graph of the impact of bit time synchronization betweenadjacent subcarrier channels;

FIG. 12 is an illustration of a two-stage crossbar switch of the DSXC ofthe present invention;

FIG. 13 is an illustration of how various network segments (areas) maybe interconnected using the present invention;

FIG. 14 is a block diagram of the present invention adapted to performan optical add/drop operation; and

FIG. 15 is a block diagram of the present invention adapted to performan optoelectronic add/drop operation.

DETAILED DESCRIPTION

As required, detailed aspects of the disclosed subject matter aredisclosed herein; however, it is to be understood that the disclosedaspects are merely exemplary of the invention, which may be embodied invarious forms. Therefore, specific structural and functional detailsdisclosed herein are not to be interpreted as limiting, but merely as abasis for the claims and as a representative basis for teaching oneskilled in the art how to variously employ the present invention invirtually any appropriately detailed structure.

Certain terminology will be used in the following description forconvenience in reference only and will not be limiting. For example, up,down, front, back, right, and left refer to the invention as orientatedin the view being referred to. The words “inwardly” and “outwardly”refer to directions toward and away from, respectively, the geometriccenter of the aspect being described and designated parts thereof.“Forwardly” and “rearwardly” are generally in reference to the directionof travel, if appropriate. This terminology will include the wordsspecifically mentioned, derivatives thereof, and words of similarmeaning.

Current telecommunications networks rely on multiple technologies tosend and route optical or electrical communication signals to a desiredlocation. Referring to FIG. 1, a prior art multilayer network 20 has anIP layer 22 which utilizes routers to deliver packets of informationend-to-end across the network, an OTN layer 24 which utilizes DXCs toswitch fixed time division multiplexed channels to create end-to-endcircuits across the OTN network 24, and a WDM layer 26 which utilizesoptical cross-connects (e.g., ROADMs) to switch wavelength signals tocreate end to end optical circuits. In recent years, an electro-opticOFDM concept has been applied to introduce orthogonality betweenadjacent optical carriers. These mutually frequency-locked orthogonaloptical carriers can be generated by optical comb generators. Similar toWDM, electro-optic OFDM is an optical domain solution known as SpectrumSliced Elastic Optical Path Network (SLICE), in which each data channelis encoded into an optical carrier through a dedicated modulator. Forsolutions requiring a large number of end-user circuits to be createdwith a relatively low data rate (such as 1 Gbps per channel) foracceptable bandwidth granularity, such an optical domain solution maynot be optimal.

IP routers 28 offer packet switching control, achieving efficientstatistical multiplexing of the available network resources across theuser population. Optical layer cross-connects (OXC) 30 offer wavelength(or lambda) switching, i.e., lightpaths or circuits of light can beswitched end-to-end across the optical network layer 26. The capacity ofthe optical circuit is fixed and set to the transmission rate availableat the physical (fiber optics) layer, e.g., 10 Gbps, 40 Gbps, or 100Gbps. Traffic grooming, consisting of multiple distinct circuits beingcombined together to form a single entity to be routed across thenetwork, is provided by the OTN layer 24 placed between the IP layer 22and the optical layer 26 to offer fine bandwidth granularity to thelinks connecting routers 28.

Referring to FIG. 2, current OTNs (such as SONET/SDH) using DXCs utilizeTDM circuit switching. TDM circuit switching is used to createend-to-end circuits with sub-wavelength bandwidth granularities and canbe provisioned to interconnect routers or other add-drop multiplexingdevices. The capacity of the OTN circuits is fixed and set to standardrates, e.g., 0.625 Gbps, 2.5 Gbps, or 10 Gbps. As shown in FIG. 2, DXCswitch 32 receives data frames along three input ports 34,35,36. Eachport receives the data frames transmitted on either an input fiber or ona wavelength of an input fiber, lambda1, lambda2, lambda3. The DXCswitch 32 monitors each arriving data frame and, based on the routinginformation provided, each data frame is switched across the DXC to thecorrect output port 37,38,39. FIG. 2 illustrates the functionalities ofthe DXC switch 32 and the need for time padding or buffering to receive,read, and re-order the data frames before sending them to the correctoutput port.

From a power consumption standpoint, electronic processing oftransported data, required in both OTN DXC and IP MPLS-TP routers,consumes significantly higher electrical energy compared to opticalcircuit switching performed by OXC which does not require electronicprocessing of optically switched data.

FIG. 3 is a graphical representation of energy consumption (Watts per Gbof switched data) for a typical electronic DXC 40 based on currenttechnology. The optical transponder unit and the line card consume about5.6 W of power which constitutes 52% of the total energy consumption.Digital cross-connector fabric consumes 0.4 W/Gbps, which is 3.8% oftotal power. An estimated 35% of the overall energy consumption is dueto cooling, such as blowers and fans, and circuit inefficiency.

Another available transport option is MPLS Transport Profile (MPLS-TP).MPLS-TP is a profile of MPLS, which is designed for use as a networklayer technology in transport networks. It is a connection-orientedpacket-switched (CO-PS) solution and offers a dedicated MPLSimplementation by adding mechanisms that provide support of criticaltransport functionality. MPLS-TP is to be based on the samearchitectural principles of layered networking that are used inlongstanding transport network technologies like SDH, SONET, and OTN.The consumption for an MPLS router is similar to the one required in theOTN DXC using existing technology.

In general, the IP routers are the most flexible and most expensivesolution that is used in the access and at the edge of the core networkwhere packets are classified at the ingress IP router and sent overpre-provisioned circuits to reach the egress IP router. The OXCs offer acost effective solution to provision such circuits with the additionaladvantage offering built-in fast protection schemes against networkelement failures (5-9 s reliability). However, they can only offerend-to-end optical circuits with the large granularity of an entirewavelength channel (e.g., 10 Gbps, 40 Gbps, or 100 Gbps). Both OTN DXCsand MPLS-TP routers are existing technologies offering end-to-endcircuits across the core network with fixed (the former) or variable(the latter) capacity, that achieve sub-wavelength bandwidthgranularity, along with fast protection schemes (5-9 s) and a cost perswitched byte of data that is favorable compared to IP routers. From apower consumption standpoint, electronic processing of transporteddata—which is required in both OTN DXC and IP MPLS-TP router—consumessignificantly higher electrical energy compared to optical circuitswitching performed by OXC (by an estimated factor of ten).

Considering that telecommunications networks are estimated to useapproximately 1% of the world's energy produced each year, solutionshave been proposed that reduce power consumption in a number of networkarchitectures. The most effective of these solutions targets IP/MPLSrouters, recommending a reduction of the packet rate processed in therouter or even the complete switching off of some IO cards at times whenthe offered load is relatively low in the network. For comparison, inthe optical domain, energy consumption is already relatively lowcompared to the electronic layer, and can be further reduced byswitching off an entire wavelength channel, or even the full set ofwavelength channels of a single fiber, which allows the in-line fiberamplifiers to be switched off during low traffic periods. However, theserecent solutions do not address the power consumption that takes placein the OTN. In fact they do not even reduce the power consumption thatis required to maintain the wavelength transmission link between fullyfunctional nodes, besides offering the transmission card switch on/offoption already mentioned.

The present invention provides replacement technology for the OTN layerwhich is responsible for a significant fraction of the energy dissipatedby today's transport networks. The invention is generally referred to asDigital Subcarrier Optical Network (DSON). Referring to FIGS. 5 and 6, amultilayer network 50 of the present invention has an IP layer 52, aDSON layer 54, and a WDM layer 56, wherein, in the OTN layer 54, thepresent invention replaces TDM based OTN/SONET/SDH with FDM-based DSCMand provides an innovative DSXC architecture 60 that can significantlydecrease the power consumption at the OTN layer 54 while maintaininggood spectral efficiency, channel granularity, data rate flexibility,and circuit switching speed. In DSON, switching and routing ofsub-wavelength channels are performed in the frequency domain (ratherthan in the time domain of prior art OTN) using orthogonal subcarrierchannels. More specifically, the present invention has the potential to(1) significantly reduce power consumption at the cross-connect by usinga circuit switch architecture which eliminates both the forwardingengine and the electronic buffering of the transferred data in currenttransport solutions, and reduces power consumption at the cross-barswitch, as shown in FIGS. 4, and (2) offer an adjustable transmission(line) rate at the sub-wavelength level (rather than the wholewavelength transmission rate) using fast electronic-domain frequencyswitching (as opposed to all-optical switching in WDM) to moreefficiently support the offered amount of traffic with closely matchingtransmission rates in the provisioned circuits. It will be appreciatedthat the components of the DSON can be used for various otherapplications. Moreover, the DSXC architecture can be fabricated invarious sizes and from a wide range of suitable materials, using variousmanufacturing and fabrication techniques.

Primary points of high electrical power consumption in routers are theforwarding engine, digital buffer, and packet switch fabric, while forDXCs they are the electronics in the line cards. DSON makes use of DSCM,a technology that multiplexes a large number of relatively low data ratesubcarrier channels into a high capacity wavelength channel. Eachsubcarrier channel carries data intended for its own destination. In amanner similar to an OXC, the DSXC crossbar switch sets up statictraffic paths from input to output ports, arid a low consumptioncrossbar switch performs cross-connection operation at baseband. Boththe forwarding engine and the digital buffer requirements of routers andDXCs are eliminated in the DSXC architecture 60. Digital signalprocessing (DSP) is used to perform DSCM at high data rates. FIG. 4shows the projected power consumption distribution across the DSXCmodules, which can be directly compared to the consumption of theelectronic DXC modules shown in FIG. 3. The reported consumption of eachmodule in DSXC is based on available data sheets, anticipated powerconsumption for the control plane based on existing control planeproducts, blowers, and power inefficiency levels that are similar tothose of already existing networking equipment. As an example, for aparticular implementation the overall power consumption of DSXC isprojected to be around 3.59 W/Gb, which offers a reduction factor of 3compared to the ˜10 W/Gb of both core IP routers and SONET equipment.

In addition to reducing energy consumption, the DSON transportarchitecture 54 of the present invention offers the following additionaladvantages (which are discussed in detail below): (1) high spectralefficiency, when compared to traditional SCM solutions, due to theorthogonality and digital isolation between subcarrier channelscrosstalk between them can be minimized without the need for a spectralguardband; (2) fast switching speed using an electronic cross-bar switchwhen compared to an all-optical cross-connect; (3) signal robustnessagainst optical transmission impairment, which may entirely circumventthe use of chromatic dispersion and PMD compensators in the opticallayer; (4) direct access to individual subcarrier channels for trafficmonitoring and add/drop functionalities, with line rates that span from1 Gbps to 100 Gbps and beyond; (5) common functionality of the DSONlayer 54, e.g., fast protection switching and rerouting of subcarrierchannels upon network element failure detection; (6) fast provisioningand switching of subcarrier circuits (˜100 ns) in the DSXC 60 withoutbeing adversely affected by the signal transient instability that mayoriginate in the optical layer; and (7) flexible data rate of eachsubcarrier to support a variety of concurrent bandwidth/capacity enduser circuit requirements.

In conventional analog-based WDM and SCM systems, adjacent channels mustbe separated by a guard band to avoid inter-channel crosstalk, andtherefore the optical bandwidth is not fully utilized. However,digital-based SCM (DSCM) does not require guard bands and can beimplemented in a number of different ways. OFDM is one such way in whichfrequency spacing between subcarriers is equal to the symbol ratecarried by each channel, and spectral overlap is allowed (shown asreference numeral 67 in FIG. 6). Orthogonality between adjacent channelsis used so that digital integration over a bit period is able to removeinter-channel crosstalk. Another example of DSCM is N-WDM in whichNyquist filters are used for spectral shaping. The sharp edge of theNyquist filter ensures the minimization of spectral overlap betweenadjacent subcarrier channels and to minimize crosstalk between them(shown as reference numeral 68 in FIG. 6). The transfer function of theNyquist filter also ensures the minimum inter-symbol-interference (ISI)for the signal. Both OFDM and N-WDM are enabled by DSP algorithmsimplemented using high-speed digital electronics. Such advanced DSPalgorithms can also be designed to allow for the compensation of varioustransmission impairments such as chromatic dispersion and PMD. AlthoughOFDM, N-WDM, and similar signal processing techniques may be used forDSCM, OFDM is used in the following explanation of the operation ofDSXC.

By virtue of the distinct OFDM subcarrier channels (each withsub-wavelength bandwidth granularity) carried by the optical signal,cross-connection operations of such channels are facilitated as follows.Referring to FIG. 6, the DSXC architecture 60 and operating principle isillustrated wherein each wavelength signal carries u orthogonalsubcarrier channels. An OFDM receiver 62 detects the incoming opticalsignal at lambdai and decomposes it into u baseband RF outputs ci1, ci2,. . . ciu. Data packets on each subcarrier are arranged such that theyall have the same destination node, and therefore, each subcarrierchannel does not have to be decomposed into individual packets/frames(which would require buffering and re-grouping operations as in a TDMcross-connect). If there are W wavelength channels coming into anddeparting from the DSXC, the RF crossbar circuit-switch 64 can performthe desired cross-connection. After the crossbar switch 64, eachsubcarrier is sent to a transmitter 66, assigned a new subcarrierfrequency, and regrouped according to the destination, and modulatedonto an outgoing wavelength signal.

The use of electronic processing to compensate for optical dispersionbuilding up along the optical fiber has gradually replaced the use ofoptical domain dispersion and PMD compensation. CMOS electrical signalprocessing capabilities built into commercial optical transceivers cannow be utilized to perform OFDM operation. In addition, coherentdetection has become practical and adopted by the telecommunicationsindustry. The power consumption estimation for such signal processing isbased on an off-the-shelf coherent 46 Gbps QPSK transceiver equippedwith DSP Agile engine. With proper modification, ADC, DAC, and DSP inthis type of digital optical transceiver can be readily reconfigured toperform OFDM operation. The embodiment of the present invention shown inFIG. 6 incorporates DSXC using digital transceivers based on coherentdetection. As an example, the power consumption of a particularimplementation of a 46 Gbps coherent transceiver (62 and 66 in FIG. 6)includes 3.5 W for coherent O/E and 22 W for coherent E/P conversion, 11W for ADC and 10 W for DSP on the receiver side, and a similar 10 W forDSP and 11 W for DAC on the transmitter side. An additional 10 W isbudgeted for serial-to-parallel conversion in the receiver andparallel-to-serial conversion in the transmitter. Additionally, 80%power efficiency is assumed for the onboard DC power supply in thetransceiver. This brings the total power consumption of a 46 Gbpscoherent DSCM transceiver to approximately 97 W, or 2.1 W/Gb. Thisincludes O/E/O and the DSP agile engine shown in FIG. 4. The specificpower consumption is expected to be further reduced with improvement inCMOS technology. A distinct advantage of using digital transceivers isthat the accumulation of noise, crosstalk, and distortion can beavoided, which can be important in multi-hop optical networks withmultiple cross-connection nodes. Cross-connect switching performed inthe electronic domain and at the subcarrier level ensures great speed,control flexibility, and bandwidth granularity.

In a traditional OFDM system, a single data stream is first mapped intoa 2-D array row-by-row, and an IFFT is performed such that each columnbecomes a subcarrier channel. In this way an OFDM symbol is usuallypartitioned into different subcarriers, which are transmitted inparallel, each at a transmission rate which is a fraction of the datastream rate. In the corresponding OFDM receiver, an FFT process is usedto convert the 2-D data array back into frequency domain and theoriginal data stream is reconstructed through parallel to serialconversion. In this process, the entire set of subcarriers is handled asa whole, as each subcarrier is the bearer of one fraction of the datastream. Conversely, the OFDM transceiver in the cross-connectarchitecture of the present invention allows for the selection ofindividual subcarrier channels to carry distinct data streams. In thiscase, each input data stream is directly mapped onto a subcarrier, andno FFT is required in the receiver.

In one example implementation of the present invention, a CIENA 10 GbpseDCO transceiver, originally designed for electronic domain dispersioncompensation, was reconfigured into an OFDM transmitter. Thistransmitter has an on-board 22 Gbps DAC with digital interface. An IQmodulator was added to enable both intensity and phase modulation. Atthe receiver, any subcarrier channel can be selected by tuning thewavelength of the local oscillator (LO). A 90° optical hybrid was usedbefore the photodetector to separate the I and Q components. 75 kmstandard single-mode fiber was used between the transmitter and thereceiver. FIG. 7 shows an example of the measured optical spectra 80obtained using DSCM: signals of subcarriers C1, C5, and C10 eachcarrying 2 Gbps QPSK data. The total optical bandwidth on thiswavelength is 10 GHz, and the total data rate is 20 Gbps. FIG. 8 showsthe measured BER vs. optical carrier-to-noise ratio (OCNR) 82 withdifferent numbers of subcarrier channels. There is negligible increaseof OCNR penalty when the number of subcarrier channels increases from 1to 5 where all 5 subcarrier channels are located on the lower sidebandwith respect to the center optical carrier. When the other 5 subcarrierchannels on the upper sideband of the spectrum are added to make thetotal number of channels 10, an approximately 1 dB OCNR penalty isintroduced. This can be partly attributed to the imperfect sidebandsuppression in the single-sideband modulation process. While there isnegligible OCNR degradation introduced by the 75 km transmission fiber,the 1.5 dB OCNR degradation of the measured BER when compared to thenumerical simulation of the same system can be attributed to pass-bandripples in the RF amplifiers, multi-pass reflections in the opticalsystem, and time jitter in the receiver.

This DSCM system is highly flexible because the receiver can select anyone or multiple subcarriers without changing the transceiver hardware.Channel selection is achieved by tuning the optical LO to the desiredsubcarrier frequency in the received optical spectrum, and coherent IQdetection translates the optical spectrum to the electrical domain. Allsubcarrier channels within the receiver electrical bandwidth can bedetected individually and crosstalk between them can be removed throughdigital processing. FIG. 9 shows the simultaneous detection 84 ofchannels C6 through C10 when the LO was set at the central frequency ofchannel C9. The BER is measured using an OCNR of 1.5 dB. No significantperformance variation was found among these channels. FIG. 10 showsresults 86 when the LO wavelength was set to the center of the signaloptical carrier in an attempt to detect all ten subcarrier channels. Theresults show reasonably uniform BER performance except for the twooutmost channels C1 and C10. The increased BER in these two channels isdue to the bandwidth limit of the particular receiver which is only 6GHz, thus the spectra of channel C1 and C10 are already partiallyoutside the receiver bandwidth. The impact 88 of bit timesynchronization between adjacent subcarrier channels is shown in FIG.11. Bit time misalignment may be kept below ±20% of the bit length T toavoid significant BER degradation.

DSCM is a generalized form of OFDM in which digital electronics is usedin the transmitter to generate coherent subcarriers and each subcarriercarries an independent data stream. In comparison to optically generatedsubcarriers, it is easier to ensure precise frequency spacing betweensubcarriers when they are generated digitally.

In the DSXC illustrated in FIG. 6, data bits carried by differentwavelengths may not be synchronized. If the RF switch fabric does notprovide a retiming function after the switch and regrouping, bit-timemisalignment between adjacent subcarrier channels may cause BERdegradation as indicated in FIG. 11. In addition, the electricalbandwidth of the OFDM receiver 62 has to be wide enough to include thespectral sidebands of each subcarrier channel, otherwise crosstalkcancelation would be incomplete. Narrowband Nyquist filters may besuitable to spectrally separate subcarrier channels and eliminate thecrosstalk. This type of digital filter may not be feasible in theoptical domain, but it can be realized in digital electronics. This alsoeliminates the need for bit-time synchronization.

In the DSXC architecture 60, different numbers of subcarrier channelscan be bundled together and switched to the same destination. Thiscapability of mixed data rate provides additional flexibility in anoptical network. Furthermore, because subcarrier channels are generateddigitally, high order modulation formats such as M-PSK and M-ary withM>4 can be used to further improve spectral efficiency if desired.

In addition to DSCM transceivers, another building block in the DSXCarchitecture 60 is the electronic crossbar circuit switch 64. A numberof different types of switches exist, including analog RF switches; andregenerative digital switches. The choice depends on the desired numberof subcarrier channels, data rate, switching speed, and maturity of thedevice technologies.

One way to realize a crossbar switch is to use analog RF circuits. Forexample, a Honeywell HRF-SW1031 1×6 RF switch device consumesapproximately 0.1 mW power with 2 GHz bandwidth per port. Six units of1×6 RF switches can be combined to make a 6×6 cross-connect, consuming0.6 mW overall. A large scale Shuffle-net with k columns and p^k rowscan be constructed using 6×6 switch building-blocks (p=6). To supportM=p^k +1 channels, the required total number of 6×6 switches is N=k·p^k.If each subcarrier channel has 1 Gbps capacity, a 100 Tbps DSXC willneed 105 ports. This requires approximately 9×104 units of 6×6 RFswitches, consuming 54 W quiescent power, which is only 0.54 mW/Gb.Dynamic power consumption, on the other hand, depends on how frequentlythe switch has to be reconfigured. For RF-based analog switches, thedynamic power is usually negligible. Although analog RF crossbarswitches use minimum electrical power, and the power consumption isindependent of the data rate of each port, realization of RF crossbarswitches with a large port count may be limited by crosstalk and powersplitting loss.

With the recent advances in CMOS electronics, large scale crossbarswitches based on CMOS circuits have become commercially available. Thistype of switch provides retiming and reshaping of the signal waveforms,thus compensating for inter-channel crosstalk and power splitting loss.For example, the Vitesse VSC-3140 chip is a non-blocking any-to-anyswitch with 144 input and 144 output ports. The bandwidth of each portcan be as high as 4.2 Gbps with an electrical power consumption ofapproximately 16 W. Using the switch at full bandwidth, the total chipswitching capacity is approximately 600 Gbps and the power efficiency is26.5 mW/Gb. To scale up switching capability, multiple VSC-3140 chipscan be combined to form a multi-stage switch fabric. For example, a2880×2880 non-blocking switch network can be constructed using 80VSC-3140 chips arranged in 3 layers (20; 40; 20). In this case, thetotal switching capacity can reach 10 Tbps with a power efficiency of100 mW/Gb, which is still two orders of magnitude less than theconsumption of the TDM-based DXC.

In the CMOS-based regenerative crossbar switch, the major powerconsumption is caused by changing the state of the flip-floprepresenting each data bit, and is, therefore, linearly proportional tothe actual traffic volume. Unlike typical TDM-based DXC, this solutiondoes not require memory/buffers and digital shift registers for datahandling and re-grouping.

FIG. 4 shows the estimated power consumption of the crossbar switchfabric to be 58 mW/Gb. This result was obtained from the datasheet ofthe 144×144 Vitesse VSC3144 chip, which consumes 21 W for a totalbandwidth of 144·10 Gbps. A larger 2880×2880 switch can be built usingthree stages of 20:40:20 VSC3144 chips, which provides a total capacityof 28.8 Tbps with 1680 W total consumption (hence the 58 mW/Gb). Ingeneral, the architecture of the switch fabric can be modularized. Ifthe traffic volume is reduced, the number of subcarrier channels can bereduced, and a certain percentage of switching units can be switched offto further reduce energy consumption.

DSXC architectures can be designed to address even larger cross-connectsolutions as illustrated in the following example. Let F be the numberof fibers reaching the network node, W the wavelengths per fiber, and Othe number of orthogonal frequencies (subcarrier channels) perwavelength. For reasonable values of these three parameters, e.g., F=9,W=40, and O=100, the total number of subcarrier channels available atthe node greatly exceeds the 3 stage, 2,880 available crossbar size. Anumber of crossbar modules are then combined together to cross-connectthe whole set of frequencies.

Assume that M crossbar modules are interconnected to form the DSXC asshown in FIG. 12, with Md decentralized modules 96, each connected to Cdincoming and Cd outgoing subcarrier channels, and M−Md=Mc centralizedmodules 98 being used to interconnect the Md decentralized modules 96.Each centralized module 98 has Cc input ports and Cc output ports. Onaverage, Cc/Md of these ports are assigned to interconnect thecentralized module 98 with one decentralized module 96. Note that thereare at least two options to cross-connect an incoming subcarrier channelas shown in FIG. 12: (i) both the input port and the output port of thechannel belong to the same decentralized module 96, and (ii) the inputport and the output port of the channel belong to distinct decentralizedmodules 96, in which case one centralized module 98 is required tocross-connect the channel from one decentralized module 96 to the other.Option (i) may be is preferred over (ii) as it reduces the total numberof crossbar modules required (and the total amount of energy dissipated)in the DSXC. Assuming that the fraction of channels that requires option(ii) is x, the total number of required crossbar modules isapproximately:M _(c) =F·W·O·x/C _(c)M _(d) =F·W·O·(1+x)/C _(d)

As discussed, at least two options are available when cross-connecting achannel: (i) only one decentralized module 96 is required, or (ii) threemodules, one centralized 98 and two decentralized 96 are required. Thecomputation of routing and frequency assignment (RFA) for end-to-endcircuit requests is important in ensuring that the value of x (andamount of energy dissipated) is minimized. In some instances, solvingthe frequency assignment problem is an effort related to finding thesolution to the improper coloring of a conflict graph.

The following is an example to illustrate how the centralized module isused for routing in DSON. All other combinatorial options of assigningfiber links, wavelengths, and subcarrier channels to the DSXC IO portsare also possible and should be considered as derivative cases of thefollowing example. Assume that the network is modeled as a graph G(N,L),with N being the set of |N| nodes and L being the set of |L| links inthe DSON. Without loss of generality, assume that every node is equippedwith the same DSXC architecture 60 shown in FIG. 12. Let D(n,d) indicatethe d-th decentralized crossbar module at node n=1,2, . . . |N|. Assumethat the decentralized modules 96 are connected via the network links inL to form Md subnets as follows: modules D(n,i) n=1,2, . . . , |N| areinterconnected and belong to Subnet i. If the circuit request is routedusing only the crossbar modules of one subnet, its assigned subcarrierchannel is cross-connected using option (i). If a circuit request isrouted using two or more subnets, each time its subcarrier channelchanges subnet, option (ii) is implemented at one DSXC node. Assume thatthe routing of each circuit request is given, i.e., a path consisting ofan ordered subset of nodes in N. A conflict graph is then created inwhich each vertex represents one of the circuits, and a pair of verticesis connected by an edge if the two corresponding circuits share at leastone common link in L. The conflict graph can be improperly colored usingexisting algorithms to assign each vertex a color such that at most kneighboring vertices are colored with the same color, while attemptingto minimize the number of required colors overall (the chromatic numberof the conflict graph). By choosing k to be the maximum number ofsubcarrier channels that can be supported by one decentralized crossbarmodule, all vertices (circuits) colored with color i will be assigned toSubnet i, and the chromatic number will indicate how many subnets (Md)are required to avoid the use of centralized crossbars. A variant of theproblem is when Md cannot be as large as the chromatic number of thegraph. In which case, the vertices (circuits) colored with any color upto Md will be routed using option (i), and the other circuits will berouted using option (ii), i.e., will make use of two or more subnets anduse some centralized crossbar modules. As a special case (k=1), theproblem of minimizing Mc is equivalent to the problem of minimizing thenumber of wavelength converters when solving the routing and wavelengthassignment (RWA) problem for a set of lightpath requests in a WDMnetwork.

In an optical network, different subcarriers may travel over differentdistances, and pass through different numbers of network nodes(cross-connect switches) as chosen by the network routing algorithms.This may result in relative delay and difference in data quality. Inorder to equalize their transmission performance, it may be desirable toapply unequal power provisioning as well as different levels ofmodulation (such as M-ary) for different subcarriers at cross-connectswitch nodes.

From a network standpoint, a cross-connect has to provide fine enoughgranularity to satisfy users with varying bandwidth demands. With thetrend of rapidly increasing capacity demand, SONET cross-connectswitches are moving to higher base rates, from OC-3 to OC-12 and even toOC-48, which simplifies the transceiver MUX and DEMUX complexity.Similarly, for the DSXC 60 of the present invention, a higher subcarrierdata rate reduces the number of subcarriers for a particular overallcapacity. This will help decrease both the required port-count of theswitch fabric and DSXC power dissipation. Although a single data ratefor all subcarrier channels would imply simple system architecture,mixed data rates will allow for tradeoffs between traffic demand, powerdissipation, and system complexity, and thus provide more options whenoptimizing overall network performance.

A hybrid data rate is possible with DSXC if the data rates are integermultiples of the lowest rate. In this case, bandwidth efficiency may bereduced and the integration at the receiver 62 may have to be performedover the longest bit period. On the other hand, if Nyquist filters areused to spectrally separate subcarrier channels, this integration maynot be required, but the digital bandpass filters with sharp edges mayrequire longer delay taps and thereby make CMOS realization moredifficult. These and other equivalent techniques can be leveraged toobtain the right compromise in DSON.

Routing and orthogonal frequency assignment (RFA) algorithms will choosewhich subset of network resources (subcarrier channels) are reserved forthe incoming end-to-end circuit requests. The optimal RFA solution is ahard problem (the routing and wavelength assignment (RWA) problem can beseen as a special case of RFA, and it is known to be NP-hard).

Another advantage of the present invention is its versatility andapplicability to a variety of network segments and technologies, apartfrom the already described conventional scope of the transport network.Referring to FIG. 13, the DSXC 64 concept can be generalized to those ofDigital Subcarrier Border Cross-Connect (DSBC) 100 and DigitalSubcarrier Gateway (DSG) 102. DSBC 100 and DSG 102 may be deployed tobridge network segments making use of similar or different networktechnologies. For example, a Metro Area Network (MAN) 104 and a WideArea Network (WAN) 106 may comprise the same DSON architecture. The twoareas can then be interconnected using DSBC 100. Thus, subcarriercircuits can be created end-to-end across the two areas, being switchedfrom one to the other by the DSBC. Note that the MAN 104 and the WAN 106need not have to share the same number of wavelengths, nor the samenumber of subcarrier channels. As long as the deployed DSCM technologiesare consistent across the two areas, circuits will be establishedend-to-end regardless of the specific resources deployed in the twoareas. Another example is the bridging of an access network, such as aPassive Optical Network (PON) 108 with the MAN 104. The PON 108 and theMAN 104 may be using different optical technologies, with the formerusing short span optical transmission over a tree topology, and thelatter using regional optical transmission gears to create either a meshor ring topology. Regardless of the different technologies used in thetwo network segments, the DSG 102 can be deployed to interconnect thePON 108 and the MAN 104, enabling subcarrier channels to be establishedacross the two segments seamlessly. For this operation to take place,the PON 108 and the MAN 104 need only to have compatible DSCMtechnologies embedded into their network equipment. The two describedexamples can even be combined, envisioning a network-wide system inwhich a subcarrier channel can be established between two users, withthe two users being connected to two geographically separated PONs,which in turn are connected to two distinct MANs, which in turn areconnected to a common WAN 106. DSCM technologies such as OFDM are beingtested across a number of possible applications, including PON and longhaul systems. Their deployment in the near future in their respectiveareas will create the right conditions for deploying the internetworkingfeatures offered by both DSBC and DSG. The present invention has thepotential to be the enabler of a new type of network service involvingthe provisioning of a dedicated (subcarrier) channel between end-userson demand. This new service would not replace IP, but rather it wouldcomplement it in various ways. In a partial summary of certain features,the present invention uses DSCM or equivalent digital signal processingto enable spectrally efficient multiplexing capability of orthogonalchannels (circuits) in optical communications and networking. Solutionsinclude fiber based networks as well as free space optics (e.g.,infrared communications). Individual channels or a group of channels canbe independently routed across a network of nodes via DSXC nodes.Special Gateway or Add Drop Multiplexing nodes are included to providethe necessary interface for other network technologies to establish anduse the channels in the network. The transmission rate can beindividually assigned to each channel, thus offering a comb of potentialtransmission rates end-to-end across the network. Rates may be as low assub-Gbps and as high as hundreds of Gbps. For comparison, currenttechnologies like OTN, SONET and SDH can only offer ODU-0 (1 Gbps),ODU-1 (2.5 Gbps), ODU-2 (10 Gbps), ODU-3 (40 Gbps), ODU-4 (160 Gbps) andODU-Flex, with the latter offering products that are ODU-2 or greater.The frequency used for a channel (or the frequencies used for a group ofchannels) can be changed along the end-to-end path by using thespecially designed cross-connect architecture, thus giving moreflexibility to routing. The present invention can be used in conjunctionwith WDM, and the wavelength used to carry the frequency of the channel(or frequencies of a group of channels) can be changed along theend-to-end path by using the cross-connect architecture, thus givingmore flexibility to routing. The last two features can be jointlyapplied to the same channel (or group of channels). With the presentinvention, network, wide time synchronization may not be required asmultiplexing is performed in the frequency domain. For comparison,existing solutions are based on time multiplexing, and network widesynchronization is needed, e.g., OTN, SONET, SDH. Channels may beprovisioned statically, or channels may be set up and torn downdynamically at all possible time scales, down to milliseconds or less.In the DSXC of the present invention, transmitted data need not bebuffered for synchronization and cross-connection purposes, unlike inother solutions, e.g., OTN, SONET, SDH.

Furthermore, the present invention offers end-to-end circuits that canbe built across multiple network segments, including but not limited toaccess networks, Passive Optical Networks (PONs), Local Area Networks(LANs), enterprise networks, Metro Area Networks (MANS), and across WideArea Networks (WANs), including WDM networks based on ROADMs. Thepreceding network types can be combined in all possible permutations andover all types of network topologies, including but, not limited tomesh, arbitrary mesh, ring, star, and tree. End-to-end channels orcircuits can be used to interconnect IP routers, OTN, SONET, SDH andother technologies nodes, end user equipment, data center (Cloud)network interfaces, enterprise, content delivery network nodes, andresidential user equipment. The present invention can be adapted tosupport unicast, anycast, and multicast traffic, individually orcombined. Another aspect of the present invention is that it can be usedin conjunction with all types of network management, network control,and network monitoring software, as well as all types of signaling andinterfaces in order to set up, tear down, restore, reroute, etc.channels or circuits across the network. All known protection andrestoration mechanisms may be applied to the subcarrier circuits,including dedicated protection, shared protection, ring protection, fastreroute, etc. Power consumption per unit of traffic carried is expectedto be lower compared to other electronically based transport networktechnologies (OTN, SONET, SDH, MPLS-TP). In addition, when not in use,part of the electronics in the DSXC can be switched off, to reduce powerconsumption to run the network and transport the offered traffic. TheDSXC architecture can be single stage or multi-stage to offer scalablesolutions. Hierarchical multiplexing may be applied to the presentinvention to offer multiple levels of traffic multiplexing. Theadvantage of hierarchical multiplexing is to offer modular options tospan across a large spectrum of transmission rates, ranging fromsub-Gbps to hundreds of Gbps. The present invention also has applicationin free space optics and networking.

The DSXC architecture of the present invention can also be applied tooptical add/drop and optoelectronic add/drop operation. Add/dropoperations can be found in various optical networks, especially in ringand bus architectures. FIG. 14 is a block diagram of an optical add/dropoperation 114 in which n+j wavelength channels 116, lambdai1, lambdai2,. . . , lambdain, lambdai(n+1), lambdai(n+2), . . . , lambdai(n+j), aredetected by n+j optical receivers 118, where n wavelength channels carryexpress, traffic, while j channels are designated for local add/drop. Ifeach wavelength channel carries u subcarriers, there will be (n+j)*usubcarrier channels at the output of the receiver array 118. The DSXC 64is capable of switching any input subcarrier (C11 through C(n+j)*u) toany output port (D11 through D(n+j)*u). The subcarrier channels are thenregrouped and loaded to n+j wavelengths channels 120, lambdao1,lambdao2, . . . , lambdaon, lambda(n+1), lambda(n+2), . . . lambda(n+j),through an array of transmitters 122.

FIG. 15 is a block diagram of an optoelectronic add/drop operation 124,in which n wavelength channels 126, lambdai1, lambdai2, . . . ,lambdain, carrying express traffic are detected by n optical receivers128. If each wavelength channel carries u subcarriers, there will be n*usubcarrier channels at the output of the receiver array 128. If thereare j subcarrier channels needing to be added to the express traffic,and j subcarrier channels needing to be dropped from the expresstraffic, then the DSXC 64 needs to have n*(u+j) input ports and n*(u+j)output ports to route any input to any output. These subcarrier channelsare then regrouped and loaded to n wavelength channels 130, lambdao1,lambdao2, . . . , lambdaon, through an array of transmitters 132.

An advantage of the present invention is its ability to offer trafficmulticast functionalities. Multicast services are required to distributethe same data stream to a number of end-users simultaneously (as opposedto performing it sequentially using unicast). The obvious advantage isthe reduction of time to deliver the data stream to the entire set ofend-users, and the reduction of network resources (including energyconsumption) required to perform the task. By using RF or CMOS switchesin its core, the DSXC can be programmed to duplicate the incomingsubcarrier channel to form two or more outgoing subcarrier channels,which will be routed over distinct paths to reach geographicallyseparated end nodes (users). In contrast, current OTN equipment can onlyperform drop and continue operations, which can be seen as a constrainedand less efficient form of the multicast function.

An additional advantage of the present invention is its ability to offerhierarchical multiplexing of subcarrier signals. For example, at thelowest level of the hierarchy, N subcarrier channels C11, C12, . . . ,C1N may be multiplexed together to be handled as a single optical signalC1. Similarly, another group of M subcarrier channels C21, C22, . . . ,C2M may be multiplexed together to be handled as a single optical signalC2. At the second lowest level of the hierarchy, the two signals C1 andC2 may be multiplexed together to be handled as a single optical signalcarrying a total of N+M subcarrier channels. Additional levels ofhierarchy can be applied as needed. While hierarchical multiplexing isalso available in OTN, SONET, and SDH, their options are limited topredetermined values for N and M, which are typically multiple integervalues of four.

The present invention has been disclosed with a focus on fiber opticsmedia, which include, for example, mono-mode fiber, multi-mode fiber,and plastic fiber. However, the present invention may also beimplemented on other types of media. Non-optical fiber media include,for example, a variety of options from Ethernet cables, USB cables, andcoaxial cables to wireless media such as radio and free-space infraredcommunication. For example, for a computer connected to an Ethernetswitch by an Ethernet cable, the bandwidth of the Ethernet cable isentirely used to carry Ethernet packets (frames) that can be used toexchange information between the computer and the Ethernet switch. Withthe present invention, however, it is possible to divide the Ethernetcable bandwidth to form multiple subcarrier channels. The first of suchchannels can be used to carry the Ethernet frames between the computerand the Ethernet switch. The other subcarrier channels (channel two andabove) can be reserved for circuit-oriented (instead of packet-oriented)communications (i.e., a circuit can be established from (to) thecomputer using one of the subcarrier channels to (from) the Ethernetswitch). In turn, the Ethernet switch can be (re)designed to offercircuit switching functionality in addition to the already existingframe (packet) switching functionality. The subcarrier circuit couldthen be routed across multiple Ethernet switches, with each being ableto offer the circuit switching functionality, within the access networkuntil reaching a gateway into the MAN or WAN segments. In this way, alarge portion (channel two and above) of the total traffic iscircuit-switched all the way through the network, while only arelatively small portion (the first channel) of the traffic remains inthe packet switched format dedicated to signaling and TCP/IP, etc. Onceagain, an advantage of using subcarrier circuit switching over Ethernetframe switching is reduced electric power consumption for switching theexchanged data across the Ethernet switches, and a transparentend-to-end digital circuit for end clients to use as best fits theirneeds (besides the conventional TCP/IP option).

The invention claimed is:
 1. A system of interconnected networksegments, the system comprising: a first network segment incorporating adigital subcarrier multiplexing technology and having a first digitalsubcarrier cross-connect switch; a second network segment incorporatingthe digital subcarrier multiplexing technology and having a seconddigital subcarrier cross-connect switch; a digital subcarrier bridgingcomponent interposed between the first and second digital subcarriercross-connect switches such that a plurality of subcarrier circuitsextend substantially seamlessly across both the first and second networksegments; and wherein switching channels is performed in the frequencydomain.
 2. The system as set forth in claim 1, wherein the digitalsubcarrier bridging component is a digital subcarrier bordercross-connect.
 3. The system as set forth in claim 1, wherein thedigital subcarrier bridging component is a digital subcarrier gateway.4. The system as set forth in claim 1, wherein the first and secondnetwork segments each have a different number of wavelengths.
 5. Thesystem as set forth in claim 1, wherein the first and second networksegments each have a different number of subcarrier channels.
 6. Thesystem as set forth in claim 1, wherein the interconnected networksegments are selected from the group consisting of: access networks,passive optical networks, local area networks, enterprise networks,metro area networks, wide area networks, and wavelength divisionmultiplexing networks.
 7. The system as set forth in claim 6, whereinthe interconnected network segments each have a network topologyselected from the group consisting of: mesh, arbitrary mesh, ring, star,and tree.
 8. The system according to claim 1 wherein frequency spacingbetween the first and second subcarriers of the digital subcarriermultiplexing technology are to a symbol rate carried by each channel. 9.The system according to claim 1 wherein the first and second subcarriersperform sub-wave length multiplexing.
 10. The system according to claim1 wherein the bridging component provides communication between thefirst and second network segments without a digital buffer.
 11. A systemof interconnected network segments, the system comprising: a firstnetwork segment incorporating a digital subcarrier multiplexingtechnology and having a first digital subcarrier cross-connect switch; asecond network segment incorporating the digital subcarrier multiplexingtechnology and having a second digital subcarrier cross-connect switch;a digital subcarrier bridging component interposed between the first andsecond digital subcarrier cross-connect switches such that a pluralityof subcarrier circuits extend substantially seamlessly across both thefirst and second network segments; and wherein the digital subcarriermultiplexing technology is operable to carry multiple high-speed datachannels via frequency division multiplexing.